Lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. In the paper-making industry, lignin may be recovered as a by-product of the cellulose product by two principal wood-pulping processes known as a sulfite process and a kraft process. In the sulfite process, lignin is solubilized from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on an alkaline degradation mechanism causing cleavage of .beta.-aryl ether linkages in the polymeric lignin which sequentially result in chemical functions of the phenolic and carboxylic type. Kraft process lignin generally is isolated by acid precipitation from the black liquor of the pulping process at a pH below the pKa of the phenolic groups.
Depending on conditions under which the lignin is precipitated, the precipitated lignin may be either in the form of free acid lignin or a lignin salt. If lignin is precipitated at a high pH, such as about 9.5 to 10, the lignin is obtained in the form of a salt. If this lignin is further processed by washing, acidifying to a low pH, such as about 2 to 5, and further washed so as to be substantially free of salt and ash-forming ingredients, free acid lignin, known as "A" lignin, is obtained. A monovalent salt of lignin, such as an alkali metal salt or ammonium salt, is soluble in water, whereas free acid lignin and polyvalent metal salts of lignin are insoluble in water.
Lignin products are widely employed as additives in various chemical processes and compounds. Because of the high degree of chemical activity characteristic of lignin, it has been variously employed as a surfactant, extender, dispersant, reinforcement, absorbent, binder, sequestering agent, emulsifier and emulsion stabilizer, and as a stabilizing and protective colloid. Lignin is sold under the trademark Indulin.RTM. by Westvaco Corporation of North Charleston, S.C. Sulfonated lignin compounds, particularly sodium salts of lignosulfonates, are widely employed as additives and dispersants in textile dyestuffs and printing pigments, and such products have been sold for a number of years under the trademarks Polyfon.RTM. and REAX.RTM. by Westvaco Corporation. The degree of sulfonation of lignin generally determines its water-solubility at various pH levels, e. g., sodium salts of sulfonated lignins are generally water-soluble at alkaline and higher pH levels, while they are water-insoluble at lower or acid pH levels.
Lignosulfonate compounds obtained from the lignin by-product of a kraft pulping process may be produced by treatment of an aqueous solution of the same with an aldehyde compound at an alkaline pH to methylolate the lignin, followed by treatment with sodium sulfite or bisulfite at an acid pH to sulfomethylate the methyol group on the phenolic nucleus of the lignin molecule. The long side chain of the lignin molecule may also be sulfonated by treatment with a sulfite or bisulfite salt.
Napthylene sulfonate compositions suitable for use as additives and dispersants in other chemical compositions, and comparable in some ways to lignosulfonates, have been produced by sulfuric acid sulfonation of napthylene. It is known that benzene and other aromatic compounds can be sulfonated with concentrated sulfuric acid, but as the concentration of the water increases during a reaction, the rate of sulfonation decreases, the reaction rate being inversely proportional to the square of the water concentration. Reaction ceases when the acid concentration reaches a level characteristic of each compound, in the case of benzene, about 78% sulfuric acid.
Much thought has been devoted to methods of carrying out the sulfonation reaction to completion. One approach involves removal of the water as formed, the net result being substantially quantitative utilization of both hydrocarbon and acid. It is also understood that certain lignin materials heretofore have been treated with sulfuric acid, gaseous sulfur trioxide, and/or oleum at varying concentrations to sulfonate the same, but such treatments have not indicated that the sulfonated lignin products are sufficiently water-soluble to be useful commercially as additives in preparation of other products and chemical compositions. Thus, as far as is known, sulfuric acid sulfonation of lignin has not been employed to produce the water-soluble sulfonated lignin products suitable for use as dispersants and additives. This is believed due to condensation reactions occurring with creation of excess water during the sulfonation reaction which causes cross-linking of the lignin, with resultant viscosity build-up and water-insolubility of the lignin.